2. Field of the Invention
This invention relates to image forming apparatuses such as printers, copiers and facsimile sets using a photo-sensitive drum made of amorphous silicon (hereinafter referred to as a-Si).
2. Description of the Prior Art
As the electrophotographic apparatus, those based on a Curlson process are well known in the art. In this type of electrophotographic apparatus, as high voltage as 4 to 8 kV or above should be applied by wire application to a photo-sensitive drum for uniformly charging the surface thereof. With this charging, ozone and such discharge products as oxides of nitrogen and ammonium salt are generated and adsorbed to the photo-sensitive drum surface, which readily causes flow of image.
To obviate such drawback, a roller charging system has been proposed, in which a conductive roller is held in contact with a photo-sensitive drum for contact charging thereof by applying a DC voltage to the conductive roller. Even with such charging system, however, slight discharge takes place in minute wedge-like interstices present between the photo-sensitive drum and the charging roller. Actually, generation of ozone is recognized with this system, and the system does not completely obviate the drawback noted above.
In the meantime, for durability improvement and freedom from maintenance, some electrophotographic apparatuses use a-Si photo-sensitive drums. Compared to organic semiconductors, however, a-Si highly absorbs moisture, and the possibility of generation of the flow of image is far high with a-Si photo-sensitive drums.
Where an a-Si photo-sensitive drum is used, therefore, to prevent the flow of image, the drum is heated for moisture removal with a sheet heater or like heater body disposed on the back side of the drum.
However, the provision of a heater necessitates heat control means or the like and complicates the construction. Particularly, in connection with the size reduction and personalization of copiers and printers, the use of a heater seriously complicates the system.
The provision of a heater poses further problems. The heater requires a predetermined warm-up time until it is ready to permit printing after the power connection. Considerable power is therefore consumed. Moreover, when the photo-sensitive drum is heated, it is elevated in temperature up to the TG temperature (i.e., glass transition temperature) of the toner, resulting in attachment of toner to the drum surface.
To solve the above problems, various techniques have been developed, which take the photo-sensitive drum, particularly, a surface layer thereof, into considerations.
For example, Japanese Laid-Open Patent Publication No. 62-272275 shows a photo-sensitive drum having a-Si photoconductive layer covered by a surface layer, which is made of an amorphous material mainly composed of silicon (Si) and carbon (C) and containing oxygen (0), hydrogen (H) and fluorine (F) and has a dynamic penetration hardness of 300 to 1,000 kgf/mm.sup.2.
The dynamic penetration hardness is set in the above range because with a dynamic penetration hardness above 1,000 kgf/mm.sup.2 the surface layer is readily chemically affected by a high Si content thereof, thus readily resulting in the flow of image.
With a dynamic penetration hardness below 300 kgf/mm.sup.2, on the other hand, the photoconductivity of the surface layer is deteriorated due to a high C content thereof. Therefore, the residual potential is increased. In addition, the hardness of the surface layer is considerably reduced, resulting in great wear thereof due to the image copying process so that image defectiveness readily takes place.
The prior art techniques described above have the following problems.
In the first place, the prior art techniques take the sole photo-sensitive drum surface layer into considerations to realize a commonly termed heater-free photo-sensitive drum. However, the flow of image is not only generated on the surface layer, but it is determined by the relation between the surface layer and the photoconductive layer and also by the relation between the photoconductive layer via the surface layer and the processing means.
Therefore, various actual difficulties are encountered when a heater-free photo-sensitive drum is to be realized by specifying the sole surface layer thereof.
Secondly, the prior art techniques seek to eliminate the flow of image particularly under high relative humidity conditions while avoiding wear of the photo-sensitive drum surface layer. However, the flow of image stems from deterioration of the drum surface and also from attachment thereto of discharge products resulting from ozone generation as result of a discharge phenomenon when charging the drum. The deterioration of the drum surface or attachment thereto of discharge products, increases the moisture absorption capacity of the drum surface and reduces the drum surface resistance even under high relative humidity conditions. This causes leaks of electrostatic latent image in the axial direction, thus resulting in the flow of image. Therefore, irrespective of any contrivance concerning the surface layer, for instance, discharge products are accumulated on the surface layer of the drum in the use thereof, and eventually the flow of image is generated. It is thus difficult to ensure stable image formation for long time.
In order to provide a heater-free photo-sensitive drum, the applicant has proposed an electrophotographic apparatus for image forming by uniformly charging a photoconductive layer supported on a base by such means as a discharge phenomenon, then exposing the photoconductive layer to write an exposure image thereon, and developing the written exposure image by inversion development. The photoconducitve layer is an a-Si layer with a thickness of 25 .mu.m or below, preferably 2 to 20 .mu.m, and is charged to a surface potential of substantially 360 V or below (Japanese Laid-Open Patent Publication No. 7-17526).
This prior art technique, however, takes the relation between the surface layer and the photoconductive layer into no consideration.
In the meantime, comminution toners have heretofore been used. The comminution toner is prepared by cooling down a paste obtained by kneading a mixture of small particles of a resin, a coloring material, a charging controller, etc., coarsely comminuting the cooled-down paste using a hammer mill, a cutter mill, etc., and then finely comminuting the resultant particles to a particle size of 8 to 15 .mu.m using a jet mill or the like.
The comminution toner particles, therefore, have distorted shapes with raised and recessed portions. Consequently, their charging is concentrated in the raised portions. In addition, a single toner particle may be in contact with the photo-sensitive drum surface in a plurality of raised portions. In such a case, the mirror image force is increased.
As a further problem, a-Si is readily subject to a phenomenon commonly termed "fog", i.e., attachment of toner to white areas. In this phenomenon, toner is attached to the photo-sensitive drum surface by the mirror image force or like forces.
The mirror image force of the toner is greatly affected by relative dielectric constant of the photo-sensitive layer and the higher the relative dielectric constant is the greater the mirror image force is. With organic photo-sensitive materials the relative dielectric constant is about 3 to 3.5, whereas with a-Si it is as great as about 10 to 12. This is why a-Si is readily subject to the phenomenon of "fog".